Astrid is long gone now, but since it was open source, there have been a number of clones. Tasks for Android is by far the most mature, and the new plugin is available for $3, either as a standalone download from Google Play or as an in-app purchase. You’ll need either Locale or Tasker installed as well to make use of it though, but if you have either, you’ll be able to use Tasks to create to-dos, and Tasker or Locale to create notification actions within geographic regions that correspond to those to-do items. The plugin itself is also open source, and you can check out the code yourself over at GitHub if you’re so inclined.
In short, you’ll be able to get notifications about your work to-dos when Tasker knows you’re at the office, and a reminder to grab milk from the grocery store as you pass it on the way home from work. The plugin right now duplicates Astrid’s old location plugin, but the developer (who clued us in to the new plugin, thanks Alex!) noted that there’ll be new features in an upcoming release. You can grab it and try it out yourself at the links below.